(1) Field of the Invention
This invention relates to a wiring board equipped with a line for transmitting high frequency signals such as microwave and millimetric wave bands.
(2) Description of the Prior Art
As lines for transmitting high frequency signals such as microwaves or millimetric wave bands, waveguides, dielectric waveguides, strip lines and microstrip lines have been conventionally known.
However, these transmission lines have had the following problems.
The strip lines and microstrip lines have a very simple structure and are suitable for production by a laminating technology. But in millimetric wave bands having at least 30 GHz, these lines have a low transmission characteristic.
On the other hand, the waveguides have excellent transmitting chacteristics, but have a defect in respect of their large sizes. For example, a standard rectangular waveguide for 60 GHz has a long diameter of 3.76 mm, and a short diameter of 1.88 mm and is too large for use in a wiring board or a semiconductor package used to transmit microwaves or millemetric wave bands. Among waveguides, a so-called dielectric waveguide in the inside of which an dielectric body is packed has a size of 1/.di-elect cons..sup.1/2 if the dielectric constant of the dielectric body is .di-elect cons.. Accordingly, the dielectric waveguide can decrease its size by using a dielectric body having a high dielectric constant. However, since basically the outside of the dielectric body should be coated with a conductor wall, it was difficult to produce this dielectric waveguide by a laminating technology.
Japanese Laid-Open Patent Publication No. 53711/1994 discloses a waveguide, as described in FIG. 19. In this waveguide, a conductive layer 12 and a conductive layer 13 are provided respectively on the upper surface and the lower surface of a dielectric substrate 11, and the upper conductive layer 12 and the lower conductive layer 13 are electrically connected by a plurality of via-holes 14 which extend in the thickness direction in the dielectric substrate. Furthermore, a plurality of via-holes 14 are arranges in two rows, and a region surrounded by the via-holes 14 and the conductive layers 12 and 13 acts as a line for transmitting signals. By the rows of via-holes 14, a pseudo-conductor wall is formed.
Since such a prior art waveguide line can be prescribed only by processing a plurality of holes in the dielectric substrate, it has excellent advantages in small sizing and productivity. However, since pseudo-conductor walls of a waveguide in this waveguide line are composed of only via-hole rows, the mode which propagates through the waveguide is limited. Even if the distance between via-holes is fully smaller than the signal wavelength, an electromagnetic wave which the conductor wall of via-hole rows can reflect is limited to an electromagnetic wave in which the direction of the electric field is that of the thickness direction of the substrate. Thus, the propagating mode of the electromagnetic wave in the waveguide line is limited to a TEn0 (n: 1, 2, 3 . . . ) mode. In other words, a component in which the direction of an electric field is perpendicular to the thickness direction of the substrate cannot be reflected by the conductor walls due to the via-hole rows. For this reason, the transmission characteristics vary by slight variations of the signal used, or the transmission characteristics are deteriorated in a connecting portion between the waveguide line and another transmission line or a discontinuous portion such as a bent portion. Furthermore, when the frequency number is lowered, the sectional shape of the waveguide increases and simultaneously, the via-hole becomes longer. As a result, the hole processing becomes difficult.